This invention relates to a method for making an electrical contact brush by chemically plating a metal coating onto graphite powder which is then dried and molded under heavy pressure for subsequent sintering in a hydrogen atmosphere to provide a metal-graphite composite suitable for forming into a monolithic or multielemental brush for high-current collectors.
Conventional monolithic solid brushes for electrical power transfer in sliding contacts are made of graphite, carbon-graphite, or metal-graphite composites. Graphite has unique antifriction properties when used for sliding contacts but has serious deficiencies which include brittleness, low strength and relatively poor conduction. Such deficiencies may be overcome by the addition of a metal such as copper or silver into the graphite. However, a monolithic brush made from such composite materials by conventional processes will not successfully operate under extremely high-current densities, e.g., 155 amp/cm.sup.2 and high sliding speeds, e.g., 70 m/sec. with a requisite low energy loss (electrical plus mechanical), a low wear rate and a high conduction rate of electricity and heat. In such brushes, the conducting and lubricating constituents must be utilized very efficiently. Inefficient utilization of the metal conducting constituent adversely affects the lubricity of the brush.
Two techniques are generally employed to fabricate conventional metal-graphite brushes. The first is a powder metallurgy technique which involves solidstate sintering of a premolded powder mixture and hotpressing of the powder mixture. The second is a metal infiltration technique which involves pressure infiltration of molten metal into the skeletal structure of the graphite. The infiltration technique is sometimes employed for making composites consisting of a low-melting point metal and a refractory metal or material. Neither of these known techniques affords an efficient usage of the metal constituent. A uniform mechanical powder mixture of metal and graphite is difficult to achieve because of the great difference between the densities of the two constituent components. Moreover, graphite powder will smear over the surface of the metal particles, thus preventing metal-to-metal direct contact to form a continuous metal matrix upon molding and sintering. In the infiltration technique, the metal filaments in the graphite skeletal structures are not continuous when cold since the thermal expansion coefficients of the metal and graphite differ by a factor of 3 or more. Metallographs of conventional metal-graphite brushes reveal such metal discontinuities. The electrical resistivities of these commercial metal-graphite brushes are relatively too high in terms of their metal contents.
Recent studies indicate an improvement to the electrical resistivity by a copper-graphite composite consisting of sintered mixtures of copper powder and copper-coated graphite powder. Such composites were compared with a composite of sintered copper and bare graphite powders. The copper coating, up to 50% by weight, on the graphite particles was achieved by electrolytic plating. Further improvements to the copper coating technique was achieved by chemical plus electrolytic platings. Efficient plating to achieve complete coverage was limited to the use of relatively large graphite particles, i.e., 100-160 microns. However, particles of this size are much larger than the size of graphite powder for practical use in brushes. Experience with the present invention indicates that a metal content, copper or silver, in a metal-graphite brush must be higher than 50%, by weight, for useful application in high-current collection systems. Mixtures of copper powder and copper-coated graphite particles are not uniform because of large density differences. Moreover, graphite powders have a flake-like configuration and cannot be evenly plated by electrolytic deposition when agglomeration is unavoidable.